Bandwidth reduction in writeable optical data storage apparatus

ABSTRACT

Apparatus for reading data from an optical medium having improved settling time characteristics and reduced bandwidth requirements is disclosed. The apparatus includes a front facet detector for producing a front facet monitor signal from a laser light beam; first and second detectors for producing first and second data signals, respectively; and a first gain control circuit responsive to the front facet monitor signal to produce a gain adjusted front facet monitor signal. The apparatus further includes a second gain control circuit responsive to the first data signal and the gain adjusted front facet monitor signal to produce a first gain adjusted data signal; a third gain control circuit responsive to the second data signal and the gain adjusted front facet monitor signal to produce a second gain adjusted data signal; circuitry responsive to the first and second gain adjusted data signals to produce a magneto-optic data signal as a function of the difference between the first and second gain adjusted data signals; or to produce a combined data signal as a function of the sum of the first and second gain adjusted data signals and responsive to the gain adjusted front facet monitor signal and the combined data signal to produce a write-once data signal as a function of the difference between the gain adjusted front facet monitor signal and the combined data signal.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to commonly-assigned U.S. patent application Ser. No.07/740,348 filed concurrently herewith, entitled "METHOD FOR BANDWIDTHREDUCTION IN WRITEABLE OPTICAL DATA STORAGE APPARATUS" by Gage et al,the teachings of which are incorporated herein by reference.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to commonly-assigned U.S. patent application Ser. No.07/740,348 filed concurrently herewith, entitled "METHOD FOR BANDWIDTHREDUCTION IN WRITEABLE OPTICAL DATA STORAGE APPARATUS" by Gage et al,the teachings of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to optical apparatus which is capable ofreading recorded data from both magneto-optic and write-once media aswell as other types of optical media.

BACKGROUND OF THE INVENTION

Optical storage systems typically use a radiation beam generated andprocessed in an optical head to record data on and/or retrieve data froman optical storage medium. Many of these systems utilize differentialdetection in order to detect small reflected signal fluctuations in thepresence of other signal components and/or system noise. An example is aconventional magneto-optic (MO) system, in which data is stored on an MOmedium in the form of marks having a distinct magnetic orientation. MOsystems generally utilize Kerr rotation of a return beam reflected fromthe MO medium to distinguish marked and unmarked areas. The Kerrrotation produces relatively small variations in the return beam and istherefore difficult to detect without differential detection.Differential detection channels are typically provided in the MO systemby separating the return beam into two orthogonal polarizationcomponents using a polarization beam splitter. The components areincident on separate detectors, and the resulting data signals areapplied to inputs of a differential amplifier which generates adifferential MO data signal representative of the stored data.

In systems with differential detection channels, it is usually importantto maximize the common-mode noise rejection in order to ensure optimalperformance. Significant degradation in output data signalcarrier-to-noise ratio (CNR) can result if, for example, one or more ofthe elements in the differential channels do not provide substantiallyequivalent gain and/or phase variations. Prior art techniques addressingthis problem have utilized, for example, strict optical head alignmentand performance tolerances, or variable gain components in one or moreof the differential detection channels. U.S. Pat. No. 4,691,308discloses an MO system with differential detection channels and avariable gain in one channel. The variable gain is adjusted in responseto an error signal corresponding to amplitude differences between thedata signals. The variable gain adjustment attempts to reduce theamplitude difference between the data signals such that common-moderejection in the differential amplifier is improved. However, thisone-channel variable gain system is susceptible to a number of problems,including long-term drift in signal levels, variable phase shifts as afunction of signal level, and poor recovery from non-ideal conditionssuch as out-of-focus or media defects. Other problems with one-channelvariable gain systems include the inability to adequately compensate forspurious output signal modulation resulting from, for example, mediabirefringence.

Japanese Patent Publication No. 4-298836 entitled "Magneto-opticalRecording and Reproducing Device" appears to disclose an MO detectionsystem which uses a pair of level control circuits controlled inaccordance with "double refractivity information." However, this systemdoes not appear to improve common-mode rejection in differentialdetection. Furthermore, it apparently utilizes a common control signalfor both level control circuits and thus fails to solve the long-termdrift, output signal modulation and other problems inherent in theone-channel variable gain system of U.S. Pat. No. 4,691,308.

Optical systems with differential detection channels can also be used togenerate a density-type data signal from a write-once (WO) medium bysumming the two data signals. As the term is used herein, WO media areintended to include erasable phase change media and read-only media suchas compact disks (CDs) which are usually generated from a masterrecording. A system which generates a WO data signal is oftensusceptible to the effects of a number of different types of systemnoise. For example, a laser diode or other optical source used to readrecorded data can exhibit mode-hopping or other instabilities whichcause variations in the power level of the read beam. Such instabilitiescan be generally referred to as optical source noise or relativeintensity noise (RIN). RIN represents a type of common-mode noise, thatis, a noise component which is common to differential detection channelsin the optical head. As noted above, common-mode noise can besubstantially eliminated in generating an MO data signal because thedata signals are subtracted. However, common-mode noise remains in a WOdata signal in which the data signals are summed.

An exemplary technique which uses subtraction of a front facet monitorsignal to limit the effects of RIN and other types of common-mode systemnoise on a WO data signal is described in U.S. Pat. No. 5,363,363entitled "Apparatus and Method For Laser Noise Cancellation in anOptical Storage System Using a Front Facet Monitor Signal," which isassigned to the assignee of the present invention and incorporated byreference herein. One embodiment of the technique involves subtracting afront facet monitor (FFM) signal representative of the optical sourceintensity from the WO data signal in a differential amplifier. Theresulting noise reduction generally depends upon proper gain and phasematching of the data and FFM signal channels. Commonly assigned U.S.Pat. No. 5,491,682 entitled "Apparatus and Method for Controllable Gainand Phase Matching in an Optical Data Storage System with Source NoiseSubtraction," the disclosure of which is incorporated herein byreference, discloses the use of a variable gain servo loop to adjust asummed data signal as a function of an FFM signal prior to subtraction.This technique, however, cannot correct for coversheet or mediasubstrate birefringence which can affect the individual data signalsdifferently. As a result, common-mode noise rejection is limited.

Commonly assigned U.S. Pat. No. 5,537,383 entitled "Optical Data StorageSystem with Differential Data Detection and Source Noise Subtraction forUse with Magneto-Optic, Write-Once and Other Optical Media," thedisclosure of which is incorporated herein by reference, improves uponthe above-noted source noise subtraction techniques for WO signalgeneration and differential detection techniques for MO signalgeneration by reducing long-term signal level drift relative to priorart variable gain systems. One embodiment of the technique usessubtraction of an FFM signal for WO signals to reduce the effects ofRIN, as well as separately-controlled variable gain differentialdetection channels for MO signals. The gain control circuitsautomatically adjust the gain of the differential detection channels inresponse to amplitude differences between the corresponding data signalsand a fixed DC voltage. The source noise subtraction technique includesa source power control circuit which varies the gain so that the FFMsignal tracks the fixed DC voltage as well. One drawback to thistechnique is that the differential detection channels and the sourcepower control circuit are servoed to a fixed reference voltage which isindependent of signal changes. During the transition from reading towriting, the amplitudes of the FFM and differential detection signalschange from one level to another. This gain is approximately a factor offour difference or larger between the signals. Thus, a high bandwidthresponse is required to go from write to read as the output levels ofthe three circuits are continually being driven to the fixed referencevoltage. This higher servo bandwidth degrades the low frequency dataresponse.

Although the above-noted exemplary optical system provides considerableimprovement in WO and MO data signal quality, there remains a need foran optical apparatus which can provide for both differential detectionfor MO signals and source noise subtraction for WO signals with reducedamplifier bandwidth requirements.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide opticalreading apparatus with reduced amplifier bandwidth requirements.

This object is achieved by apparatus for reading data from an opticalmedium with reduced bandwidth, comprising:

(a) front facet detector means for producing a front facet monitorsignal from a laser light beam;

(b) first and second detector means for producing first and second datasignals, respectively;

(c) first gain control means responsive to the front facet monitorsignal to produce a gain adjusted front facet monitor signal;

(d) second gain control means responsive to the first data signal andthe gain adjusted front facet monitor signal to produce a first gainadjusted data signal;

(e) third gain control means responsive to the second data signal andthe gain adjusted front facet monitor signal to produce a second gainadjusted data signal; and

(f) means responsive to the first and second gain adjusted data signalsto produce a magneto-optic data signal as a function of the differencebetween the first and second gain adjusted data signals; or to produce acombined data signal as a function of the sum of the first and secondgain adjusted data signals and responsive to the gain adjusted frontfacet monitor signal and the combined data signal to produce awrite-once data signal as a function of the difference between the gainadjusted front facet monitor signal and the combined data signal.

ADVANTAGES

An advantage of the present invention is to provide apparatus forreading data from an optical medium having improved settling timecharacteristics of the data detection channels and reduced bandwidth ofthe gain adjustment circuitry.

Another advantage of the present invention is to provide apparatus forreading data from an optical medium having decreased detection channeland system noise.

A feature of the present invention is the simplification of the frontfacet servo electronics required in prior art apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus for reading data from anoptical medium having a front facet servo control to generatemagneto-optic and write-once data signals in accordance with the presentinvention; and

FIG. 2 is a schematic diagram illustrating the source noise subtractionand differential detection circuitry of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

Although the present invention will be illustrated herein primarily interms of generating magneto-optic (MO) and write-once (WO) data signalsfrom MO and WO media, respectively, it should be understood that this isby way of example and not limitation. The invention can be utilized withany of a number of other types of optical media, including ablative andwrite-once and erasable phase-change media. The invention should beunderstood as an apparatus for balancing and differential detection andreducing source noise from a sum density detection and can be applied toother apparatus such as wobbled groove signal detection and datareadout. Those skilled in the art can readily adapt the teachings of thepresent invention to these and other alternative media.

Referring to FIG. 1, an optical apparatus 10 in accordance with thepresent invention is shown which utilizes a front facet servo controland differential detection channels to generate both MO and WO datasignals. An incident radiation beam is generated by an optical source12, which can be, for example, a laser diode, a laser or an LED. Opticalsource 12 produces an incident radiation beam at a given wavelength,which will generally vary depending upon the application. An exemplarywavelength suitable for use in apparatus 10 is about 780 nanometers. Theradiation beam from optical source 12 is collimated by a collimatinglens 14, which in this embodiment can have a focal length on the orderof 8 mm. The collimated radiation beam is transmitted through apolarization beam expander/splitter 16. Alternative beam expansiontechniques could be used, such as including a circularizing lens (notshown) in the path of the incident beam, and would generally alter thedesired focal length of lens 14. A suitable circularizing lens is themodel VPS700 lens available from Blue Sky Research of San Jose, Calif.

The beam expander/splitter 16 can have an s-polarization reflectivity ofnear 100% and a p-polarization reflectivity of about 20%, indicatingthat 100% and 20% of the intensity of the s-polarized beam componentsand p-polarized beam components, respectively, are reflected by aninternal surface 18, and any remaining portions are transmitted throughthe surface 18. A portion of the incident radiation beam is thusreflected by surface 18 toward a detector focusing lens 20 and therebyfocused onto a front facet detector 22. The reflected portion of thelinearly-polarized radiation beam thus includes both s-polarized andp-polarized components, which are detected by the front facet detector22 to provide an indication of the intensity level of the incidentradiation beam. The front facet detector 22 is also referred to as afront facet monitor (FFM) or, more generally, an optical source monitor.As will be described in greater detail below, the FFM signal from frontfacet detector 22 is used in a front facet subtraction technique toreduce source noise in a WO data signal, as well as in a source powerservo loop 70 to maintain the output power level of source 12 at adesired value.

The portion of the incident radiation beam which is not reflected bysurface 18 to the front facet detector 22 passes through surface 18 andis incident on an objective lens 24. The surface 18 of polarization beamsplitter/expander 16 transmits a linear p-polarization of the radiationbeam, which is perpendicular to the s-polarized portion of the radiationbeam reflected by surface 18. The direction of the incident beam can bealtered by including additional optical elements (not shown), such as aturning prism or fully-reflecting mirror, between the beam splitter 16and the objective lens 24. For example, a turning prism or reflectingmirror could be arranged to reflect the beams by 90° to facilitateplacement of the components of apparatus 10 relative to the storagemedium in a reduced-height optical disk drive.

Objective lens 24 focuses the incident beam onto an optical storagemedium 26. Optical components 14, 16 and 24 represent only an exemplarymeans of applying an incident radiation beam to the storage medium 26,and any of a number of other arrangements of components could be used asan application means herein. The storage medium 26 typically includesdata tracks (not shown) arranged in a spiral or concentric circles on adata storage surface 26A. A portion of the storage medium 26 is shown ina side-sectional view in FIG. 1. The storage medium 26 can include atransparent substrate 26B which serves as a protective layer over thedata storage surface 26A. The storage medium 26 can also include apregroove or similar light-diffracting structure suitable for generatinga tracking error signal (TES) using known techniques. The incidentradiation beam reads data previously recorded in the form of marks onthe storage medium 26 by detecting variations in a return beam reflectedfrom the medium 26. Apparatus 10 can also be used to record data on themedium 26 by modifying the power level of the incident beam or byproviding additional recording system elements, the placement andoperation of which are generally well-known in the art.

It will now be assumed that storage medium 26 is an MO medium. Theincident beam applied to the MO medium 26 usually has a substantiallylinear polarization. Interaction with the data storage surface 26Acauses the incident radiation beam to be reflected and diffractedtherefrom. The resulting radiation beam, referred to herein as a returnbeam, generally has an elliptical polarization due to Kerr effectvariations produced at the MO medium surface. It should be noted that inother embodiments the return beam can be either reflected or diffractedfrom the medium, or even transmitted through the medium in embodimentsutilizing, for example, an optical tape storage medium. The return beamis collimated by objective lens 24, substantially reflected by surface18, and then applied to a polarization beam splitter 28 which can havean s-polarization reflectivity of near 100% and a p-polarizationreflectivity of about 75%. As a result, internal surface 30 of thepolarization beam splitter 28 transmits about 25% of the p-polarizedportion of the beam incident thereon to a detector focusing lens 32. Thelens 32 focuses the transmitted portion of the beam onto a detector 34,which can include multiple detector elements. Detector 34 can be used togenerate a TES as well as a focus error signal (FES) using well-knowntechniques.

The portion of the return beam which is not transmitted through surface30 of the beam splitter 28 is reflected by surface 30 toward a lens 36and through a general wave plate 38. The general wave plate 38 modifiesthe polarization of the portion of the return beam passing through it bya given retardance value between about 90° and 180.° The actualretardance value selected can vary with the application. Factors whichshould be taken into account in selecting a suitable retardance valueinclude optical head properties, such as phase shifts introduced by beamsplitters or other elements, and media properties such as mediabirefringence. Details regarding selection of a suitable retardancevalue for wave plate 38 to optimize the data signal CNR in a givenembodiment can be found in U.S. Pat. No. 5,282,188, which is assigned tothe assignee of the present invention and incorporated by referenceherein. Those skilled in the art will recognize that wave plate 38 canbe used with any of a number of alternative retardance values and/orplate rotations. In other embodiments of the present invention, the waveplate 38 can be replaced with a liquid crystal variable retarder, suchthat different retardance values can be readily selected depending onwhether apparatus 10 is being used with an MO or WO medium. The variableretarder can be configured in the manner described in U.S. Pat. No.5,282,188.

The portion of the return beam transmitted through wave plate 38 is thenapplied to another polarization beam splitter 40. An internal surface 42of the polarization beam splitter 40 reflects an s-polarized componentof the return beam to a first detector 44 and transmits a p-polarizedcomponent to a second detector 46. The polarization beam splittersurfaces 18, 30 and 42 can be, for example, multilayer coatings whichreflect and/or transmit desired amounts of s-polarized and p-polarizedlight. It should be noted that any of a number of alternativedifferential detection arrangements could be used in apparatus 10. Forexample, the polarization beam splitter 40 could be replaced with aWollaston beam splitter and the position of detectors 44, 46 could beadjusted such that one detector receives components of the return beamhaving a p-polarization while the other receives components having ans-polarization. As another alternative, the general wave plate 38 couldbe replaced with a phase plate selected to remove phase ellipticity inthe return beam, and the beam splitter 40 could be a rotatable Wollastonbeam splitter rotated to a fixed angle of about 45°. The beam splitter40 could also be replaced with a larger beam splitter, such as beamsplitter 30. In general, the beam splitter 40 separates the portion ofthe return beam incident thereon into first and second polarizationcomponents which are supplied to detectors 44, 46, respectively.Detectors 44, 46 generate first and second data signals from thepolarization components. Detectors 44, 46 can be, for example,positive-intrinsic-negative (PIN) or avalanche photodiodes, or any otherdevice capable of detecting incident optical radiation, includingmulti-element detectors that are used to generate servo signals oralternatively, detectors 44, 46 could be integrated. Suitablephotodiodes for detectors 44, 46 include part Nos. FFD100 and FFD200,from EG&G Optoelectronics of Quebec, Canada.

The first and second data signals from detectors 44, 46, respectively,can be used to provide a differential MO data signal which is indicativeof data recorded on the MO medium 26. In the embodiment of the presentinvention, one differential detection channel in apparatus 10 includesdetector 44, a first variable gain amplifier 50, and a first errordetector circuit 66, while the other differential channel includesdetector 46, a second variable gain amplifier 52, and a second errordetector circuit 68. The first and second error detector circuits 66, 68vary the gains of amplifiers 50, 52, respectively, in accordance withfeedback signals V_(C1), V_(C2), respectively, to generate first andsecond gain adjusted data signals, respectively. Feedback signalsV_(C1), V_(C2) are generated as a function of a gain adjusted FFM signalfrom the front facet detector 22, the gain being adjusted by a variablegain amplifier 60 in accordance with a set point voltage V_(S) from, forexample, a microprocessor-controlled voltage source (not shown). The setpoint voltage V_(S) could be fixed if, for example, all light conditionsin the optical head are well known and do not exceed the dynamic rangeof the apparatus. Alternatively, the set point voltage V_(S) could beadjustable if, for example, the light conditions are known to change dueto media reflectivity differences. Such change in light conditions canoccur, for example, in a universal head having WO and MO capability. Inaddition, if the dynamic range is inadequate between write and readfunctions, V_(S) can be adjusted to maintain the signals within thedynamic range of the apparatus. V_(S) can also be varied to optimize thedata signal amplitude for different playback conditions.

As shown in FIG. 1, the first and second gain adjusted data signals fromvariable gain amplifiers 50, 52, respectively, are applied to inputs ofa differential amplifier 54 which generates an MO data signal as thedifference between the first and second gain adjusted data signals.Other signal difference circuits suitable for generating an MO datasignal from the first and second gain adjusted data signals could beused in place of the differential amplifier 54.

It will now be assumed that storage medium 26 is a WO medium. A WO datasignal can be generated in apparatus 10 from a return beam reflectedfrom, for example, stamped CD-like media or a phase change or ablativeWO medium. The return beam is directed through optical components 24,16, 28, 36, 38 and 40 in the manner previously described in conjunctionwith the generation of an MO data signal. Detectors 44, 46 generate datasignals which pass through variable gain amplifiers 50, 52,respectively, and first and second error detector circuits 66, 68,respectively to produce first and second gain adjusted data signals,respectively. A summing amplifier 56 receives the first and second gainadjusted data signals and combines them to form a sum density-type WOdata signal, referred hereinafter as a combined data signal. Datasignals can thus be generated for both MO and WO media in apparatus 10of FIG. 1 by appropriate processing of the data signals from detectors44, 46. Alternative signal combiners can be utilized in place of thesumming amplifier 56.

Optical apparatus 10 also implements a source noise subtractiontechnique to generate the WO data signal. The uncompensated WO datasignal output of summing amplifier 56 is applied to one input of adifferential amplifier 58. The gain adjusted FFM signal output fromvariable gain amplifier 60 is applied to an amplifier 62, and theamplified gain adjusted FFM signal is then applied to another input ofdifferential amplifier 58. The amplitude and phase of the FFM and WOsignals at the input of differential amplifier 58 are equalized in amanner to be described in detail below in conjunction with FIG. 2. Theoutput of differential amplifier 58, which is the difference between theamplified gain adjusted FFM signal and the uncompensated WO data signal,is a compensated WO data signal in which source noise-inducedfluctuations are substantially reduced.

Apparatus 10 in accordance with the present invention, as shown in FIG.1, also includes a source power control circuit 70. The FFM signaloutput from the front facet detector 22 is applied as a control signalto a source driver (not shown) connected to the source 12. The sourcedriver converts the output voltage of a preamplifier 64 to a drivecurrent suitable for controlling the output power of source 12. Thecircuit 70 adjusts the source driver output and thereby the intensitylevel of the radiation beam generated by source 12. The circuit 70,source driver, source 12 and front facet detector 22 thus form a sourcepower servo loop which adjusts the source power as a function of the FFMsignal.

Any of a number of alternative techniques could be used to detectradiation from optical source 12 in order to provide an indication ofthe intensity level of the incident beam. For example, radiation from arear facet of a laser diode optical source could be detected by suitablearrangement of the front facet detector 22 in a path of the rear facetradiation. It should be noted, however, that front facet radiation oftenexhibits better linearity and repeatability, and is less susceptible totemperature variation than rear facet radiation. It should also be notedthat the FFM signal from the front facet detector 22 can be used tonormalize a tracking signal using methods well known to those skilled inthe art.

Apparatus 10 can include additional elements (not shown) suitable forprocessing the FFM signal and the data signals. For example, ananalog-to-digital converter, digital signal processor, memory, anddigital-to-analog converter can be included to obtain and processsamples of the FFM and data signals. The control signals for variablegain amplifiers 50, 52 could be adjusted in response to operatingcommands generated by the digital signal processor. This technique canbe used to adjust signal levels during recording, optical disk drivestart-up or other phases of system operation.

Referring now to FIG. 2, a schematic diagram shows the source noisesubtraction and differential detection channels using the front facetservo control of apparatus 10 in greater detail. The source noisesubtraction portion of FIG. 2 includes front facet detector 22, whichsupplies an FFM signal to preamplifier 64 to produce an amplified FFMsignal. The preamplifier 64 can be, for example, an OPA620 amplifierfrom Burr-Brown, Inc. of Tucson, Ariz. The front facet detector 22 andthe preamplifier 64 preferably have a combined bandwidth ofapproximately three times the highest data signal frequency in thesystem, although other bandwidths could also be used. The amplified FFMsignal output of the preamplifier 64 is applied to variable gainamplifier 60 to produce a gain adjusted FFM signal. The gain ofamplifier 60 is controlled by the set point voltage V_(S) as describedin conjunction with FIG. 1 above. The gain adjusted FFM signal is thensupplied via capacitor C_(F1) to amplifier 62 which doubles the gain ofthe gain adjusted FFM signal. Capacitor C_(F1) serves to block the DCcomponent of the gain adjusted FFM signal from reaching amplifier 62,while passing only the alternating-current (AC) component of the gainadjusted FFM signal to amplifier 62. Although those skilled in the artwill recognize that C_(F1) does not provide a sharp cut-off at aparticular frequency, in this embodiment, AC components are generallyconsidered to be components other than DC. In an alternative embodiment,C_(F1) can be replaced with a more complex signal filter to provide anydesired frequency response in the channel, or can be eliminated suchthat the amplifier 62 receives all signal components including the DCcomponent.

As noted above, a compensated WO data signal is generated by subtractingthe uncompensated combined data signal at the output of the summingamplifier 56 from the FFM signal supplied by amplifier 62. Maximumcommon-mode noise rejection for the compensated WO data signal output isrealized when the combined data signal matches the FFM signal in bothamplitude and phase. In the embodiment of FIG. 2, the combined datasignal is driven to a level of the gain adjusted FFM signal by first andsecond error detector circuits 66, 68 in a manner to be described below.Amplifier 62, therefore, preferably has a gain of about 2.0 such thatthe FFM signal is amplitude matched with the combined data signal at theinput of differential amplifier 58. Alternatively, the combined datasignal can be divided approximately in half prior to front facetsubtraction. The bandwidth of error detector circuits 66, 68 in the datadetection channels is preferably comparable to the bandwidth of thefront facet detector 22 and the preamplifier 64 combination, such thatwideband matching of amplitude and phase are provided for both the FFMand the combined data signal.

Apparatus 10 includes delay elements 74, 92, and 94, which are connectedbetween preamplifiers 64, 49, 48, respectively, and variable gainamplifiers 60, 50, 52, respectively, in order to provide proper phasematching of the FFM signal and the first and second data signals at theinput of differential amplifier 58. Alternatively, delay elements 74,92, and 94 can be positioned after capacitors C_(F1), C_(A2), andC_(B2), respectively. In general, the amplifiers used in the circuitryof FIG. 2 exhibit relatively linear group delay responses. A constantgroup delay can generally be considered as a fixed signal propagationtime independent of frequency. Thus, the FFM and combined data signalchannels can be phase equalized using a passive analog delay lineproviding a fixed amount of delay. Delay elements 74, 92, 94 cantherefore be, for example, manually or electronically controlledprogrammable delay lines which provide a selectable amount of delay indiscrete increments, or multiple delay lines providing a desired totalamount of delay. One suitable device is the ten nanosecond, ten tapvariable delay line part No. 60Z14A010H from Sprague Electric Co. ofStamford, Conn. Alternatively, delay elements 74, 92, 94 can be singlefixed amounts of delay calculated to provide the desired phase matchingat the input of the differential amplifier 58. The term "delay line" asused herein refers to devices providing either fixed or adjustableamounts of delay using physical lengths of transmission line or anyother suitable delay mechanism. The amount of delay provided by delayelements 74, 92, 94 will generally vary depending upon the relativelength of the FFM and data signal paths. It is preferred that the phasedifference between the two signals at the input of the differentialamplifier 58 is reduced to a value less than about five degrees at thehighest data signal frequency of interest, although larger phasedifferences can be acceptable in a given application. Of course,reducing the phase difference to zero will produce maximum common-modenoise rejection in amplifier 58. Although delay elements 92, 94 areshown in both signal channels in FIG. 2, it should be understood thatdelay may be required in any one of the channels, or none of thechannels if matching of the FFM and data signals is adequate.

The differential detection channels in FIG. 2 will now be described. Asdescribed in conjunction with FIG. 1 above, the detection channels eachinclude a separate error detector circuit 66 or 68 in which a feedbacksignal V_(C1) or V_(C2) is generated and used to adjust the gain ofamplifier 50 or 52, respectively. The variable gain amplifiers 50, 52are adjusted such that data signal high-frequency (HF) componentsprovided to the differential amplifier 54 are substantially matched inamplitude. The gain adjustments are made by comparing direct-current(DC) and other LF components in the data signals to the gain adjustedFFM signal output from variable gain amplifier 60. Error detectorcircuits 66, 68 each function as a servo loop which matches data signalamplitudes at the input of differential amplifier 54 and therebymaximizes common-mode rejection and the output SNR of the MO datasignal. In addition, settling time characteristics of the differentialdetection channels are improved by substantially minimizing gain changeswhile going from read to write functions. Frequently when an input to aservo loop rapidly changes, there is often an initial rise or fall ofthe output of the servo, which is a transient response. By the use ofthe term "settling time characteristics" is meant the time required forthe servo loop output to settle within a predetermined accuracyfollowing the transient.

Error detector circuits 66, 68 include error detector/integrators 76,78, low pass filters (LPF) 80, 82 and feedback signal lines 88, 90,respectively. In this embodiment, variable gain amplifiers 50, 52 areconnected to detectors 44, 46 through preamplifiers 48, 49,respectively. Preamplifiers 48, 49 can be OPA620 amplifiers fromBurr-Brown, Inc. of Tucson, Ariz. In other embodiments, preamplifiers48, 49 could be eliminated and variable gain amplifiers 50, 52 couldserve as preamplifiers.

The gain adjusted data signals from variable gain amplifiers 50, 52 areeach supplied to an input of error detector/integrators 76, 78,respectively. The gain adjusted FFM signal from amplifier 60 is suppliedto a second input of error detectors/integrators 76, 78. Errordetector/integrators 76, 78 compare the first and second gain adjusteddata signals with the gain adjusted FFM signal in integrating amplifiers84, 86, respectively, to generate error signals. Low-pass filters (LPF)80, 82 are positioned between error detector/integrators 76, 78 andintegrating amplifiers 84, 86, respectively, to filter the error signalssuch that low-frequency (LF) components thereof are supplied to an inputof integrating amplifiers 84, 86. The cut-off frequency of filters 80,82 can be, for example, selected between about 10 kHz and 100 kHz inorder to minimize the effects of system noise on the source power.Amplifiers 84, 86 are configured to integrate the difference between theLF components of the detected error signals and the gain adjusted FFMsignal over time using capacitors C_(A1), C_(B1), respectively. Theintegrated error signal outputs of amplifiers 84, 86 are fed back vialines 88, 90 as feedback signal inputs V_(C1), V_(C2) to variable gainamplifiers 50, 52, respectively. Each error detector circuit 66, 68 actsto maintain an error signal of zero volts within the bandwidth of theloop. The data signals from amplifiers 50, 52 are also supplied viacapacitors C_(A2) and C_(B2) to differential amplifier 54. CapacitorsC_(A2) and C_(B2) serve to block the DC components of the data signalsfrom reaching differential amplifier 54, while passing onlyalternating-current (AC) components of the data signals to amplifier 54.It is to be appreciated that using the AC coupled signals increases thedynamic range of the apparatus. Although those skilled in the art willrecognize that C_(A2) and C_(B2) do not provide a sharp cut-off at aparticular frequency, in this embodiment, AC components are generallyconsidered to be components other than DC. In alternative embodiments,C_(A2) and C_(B2) can be replaced with more complex signal filters toprovide any desired frequency response in the channel, or can beeliminated such that differential amplifier 54 receives all signalcomponents including DC components.

Variable gain amplifiers 50, 52 are preferably matched amplifiers in adevice such as the Model No. AD602 available from Analog Devices ofNorwood, Mass. The AD602 package includes two matched, low noise,voltage-controlled amplifiers with relatively stable group delay, amaximum control bandwidth of about 1 MHz, an amplification bandwidth ofabout 35 MHz independent of gain setting, a gain scaling of about 32dB/volt and a gain range of about -10 dB to +30 dB. Of course,amplifiers with other bandwidth and gain parameters could also be used.Alternative variable gain amplifiers include the part Nos. CLC 520 orCLC 522 from Comlinear, Inc. of Fort Collins, Colo., and the part No.VCA-610 from Burr-Brown, Inc. of Tucson, Ariz. It should be emphasizedthat these are examples only, and numerous other alternatives will beapparent to those skilled in the art. Other embodiments could usedifferent types of variable gain circuits to alter signal level inresponse to an input control signal. For example, a variable attenuatorproviding a variable amount of signal attenuation in response to acontrol input is considered a variable gain circuit herein.

Low pass filters 80, 82 limit the bandwidth of the portion of the datasignal that is fed back to control the gain of variable gain amplifiers50, 52, respectively. This bandwidth limiting reduces potentialoscillations and modulation noise in the loops before the integratingamplifiers 84, 86. Modulation noise can result when HF components of thefirst and second data signals have different amplitudes or a duty cycleother than fifty percent. A mismatch in data signal amplitudes at theinput of differential amplifier 54 can then arise even though the DCand/or LF components of the data signals have been equalized by theservo loops. The magnitude of the channel gain error from modulationnoise is generally dependent on the cross-channel amplitude mismatch,the degree to which the duty cycle deviates from fifty percent, and theamplitude of the DC signal components. Filters 80, 82 can be implementedas, for example, single-pole resistor-capacitor (RC) networks. Higherorder filters could also be used, depending on the amount of filteringrequired in a given application. A single-pole filter suitable for usein a system in which the data signal HF components approach 10 MHz has a3 dB passband of about 100 kHz and reduces modulation noise byapproximately 30 dB. Specific values for the servo control loopparameters will be optimized for a given apparatus. Important factorsinclude, but are not limited to, charge content of the code, andfrequency of the media disturbances including nonuniformity, substrateand/or coversheet birefringence. The cut-off frequency of filters 76, 78is limited on the high end by the system data rate and on the low end bythe highest frequency of the system perturbations that the servo isrequired to track. For example, in certain applications, it can bedesirable for the servos to track media birefringence-induceddistortions at frequencies of up to 5 kHz or more. A suitable cut-offfrequency for use in many optical storage applications is about 100 kHz.This cut-off frequency refers to a frequency above which signalcomponents are attenuated by about 10 dB or more.

Integrating amplifiers 84, 86 are preferably low offset, low bandwidthdevices such as the OP400 amplifiers from Analog Devices of Norwood,Mass. A low offset can limit channel gain error, and a low bandwidth canprovide further filtering of modulation noise. In a preferredembodiment, the bandwidth of amplifiers 84, 86 is about six to ten timesthe cutoff frequency of low pass filters 80, 82. This value maintainsthe low pass filter passband as the dominant passband control of thesystem.

The embodiment of the invention illustrated in FIGS. 1 and 2 provides anumber of advantages. For example, the gain adjusted FFM signal is usedto servo the error detector circuits 66, 68 in the differentialdetection channels. Because the gain adjusted FFM signal is used ratherthan a fixed DC voltage, the proportion of light between the FFM and thedifferential detection channels does not change during the transitionfrom reading to writing. This permits the bandwidth of error detectorcircuits 66, 68 to be substantially reduced from the currentimplementation of prior art apparatus. The reduction of bandwidth causesa decrease in data detection channel and system noise, as well asimproves the low frequency response and the settling timecharacteristics of the data detection channels. In addition, the gainadjusted FFM signal is used to control both the error detector circuits66, 68 and the source power control circuit 70, thereby simplifyingoverall system control and processing. Furthermore, this improvedcontrol is provided without significantly increasing the cost orcomplexity of the optical head. The invention permits implementation ofa universal optical head capable of generating MO, WO, and other datasignals from a variety of different optical media. Allowing V_(S) tochange enables the data signal amplitude to be optimized.

Alternative embodiments can include operation of the error detectorcircuits using high-frequency components of the data signals, inaddition to or in place of the low-frequency components used in the FIG.2 embodiment. Those skilled in the art will recognize that the inventioncan be implemented using digital servo loops, and that integrators canbe replaced with other suitable signal processing circuits.

While the preferred embodiment of the present invention has been shownand described, it will be manifest that many additional changes andmodifications can be made therein without departing from the essentialspirit of the invention. It is intended, therefore, in the annexedclaims, to cover all such changes and modifications as may fall withinthe true scope of the invention.

    ______________________________________                                        PARTS LIST                                                                    ______________________________________                                        C1            capacitor                                                       C.sub.A1, C.sub.A2                                                                          capacitors                                                      C.sub.B1, C.sub.B2                                                                          capacitors                                                      V.sub.C1, V.sub.C2                                                                          feedback voltages                                               V.sub.S       set point voltage                                               10            optical apparatus                                               12            optical source                                                  14            collimating lens                                                16            beam splitter/expander                                          18            beam splitter surface                                           20            detector focusing lens                                          22            front facet detector                                            24            objective lens                                                  26            optical storage medium                                          26A           data storage surface                                            26B           transparent substrate                                           28            beam splitter                                                   30            beam splitter surface                                           32            detector focusing lens                                          34            detector array                                                  36            lens                                                            38            wave plate                                                      40            beam splitter                                                   42            beam splitter surface                                           44, 46        detectors                                                       48, 49        preamplifiers                                                   50, 52        variable gain amplifiers                                        54            differential amplifier                                          56            summing amplifier                                               58            differential amplifier                                          60            variable gain amplifier                                         62            amplifier                                                       64            preamplifier                                                    66, 68        error detector circuits                                         70            source power control circuit                                    74            delay element                                                   76, 78        error detector/integrators                                      80, 82        low pass filters                                                84, 86        integrating amplifiers                                          88, 90        feedback signal line                                            92, 94        delay elements                                                  ______________________________________                                    

What is claimed is:
 1. Apparatus for reading data from an optical mediumwith reduced bandwidth, comprising:(a) front facet detector means forproducing a front facet monitor signal from a laser light beam; (b)first and second detector means for producing first and second datasignals, respectively; (c) first gain control means responsive to thefront facet monitor signal to produce a gain adjusted front facetmonitor signal; (d) second gain control means responsive to the firstdata signal and the gain adjusted front facet monitor signal to producea first gain adjusted data signal; (e) third gain control meansresponsive to the second data signal and the gain adjusted front facetmonitor signal to produce a second gain adjusted data signal; (f) meansresponsive to the first and second gain adjusted data signals to producea magneto-optic data signal as a function of the difference between thefirst and second gain adjusted data signals and to produce a combineddata signal as a function of the sum of the first and second gainadjusted data signals; and (g) means responsive to the gain adjustedfront facet monitor signal and the combined data signal to produce awrite-once data signal as a function of the difference between the gainadjusted front facet monitor signal and the combined data signal.
 2. Theapparatus of claim 1 wherein the gain adjusted front facet monitorsignal producing means includes:(i) a preamplifier responsive to thefront facet monitor signal to produce an amplified front facet monitorsignal; and (ii) a variable gain amplifier responsive to the amplifiedfront facet monitor signal to produce the gain adjusted front facetmonitor signal.
 3. The apparatus of claim 1 further including delayadjustment means for adjusting the front facet monitor signal and thefirst and second data signals so that a phase of the front facet monitorsignal substantially matches the first and second data signals.
 4. Theapparatus of claim 2 further including an amplifier responsive to thegain adjusted front facet monitor signal to double the gain of the gainadjusted front facet monitor signal.
 5. The apparatus of claim 1 whereineach of the first and second gain adjusted data signal producing meansincludes:(i) preamplifier means responsive to the first and second datasignal producing means to produce first and second amplified datasignals, respectively; (ii) variable gain amplifying means responsive tothe first and second amplified data signals to produce first and secondgain adjusted data signals, respectively; (iii) error detecting meansresponsive to the gain adjusted front facet monitor signal and to thefirst and second gain adjusted data signals to produce first and secondfeedback signals, respectively; and (iv) means responsive to the firstand second feedback signals to produce the first and second gainadjusted data signals, respectively.
 6. Apparatus for reading data froma magneto-optic medium with reduced bandwidth, comprising:(a) frontfacet detector means for producing a front facet monitor signal from alaser light beam; (b) first and second detector means for producingfirst and second data signals, respectively; (c) first gain controlmeans responsive to the front facet monitor signal to produce a gainadjusted front facet monitor signal; (d) second gain control meansresponsive to the first data signal and the gain adjusted front facetmonitor signal to produce a first gain adjusted data signal; (e) thirdgain control means responsive to the second data signal and the gainadjusted front facet monitor signal to produce a second gain adjusteddata signal; and (f) means responsive to the first and second gainadjusted data signals to produce a magneto-optic data signal as afunction of the difference between the first and second gain adjusteddata signals.
 7. The apparatus of claim 6 wherein the gain adjustedfront facet monitor signal producing means includes:(i) a preamplifierresponsive to the front facet monitor signal to produce an amplifiedfront facet monitor signal; and (ii) a variable gain amplifierresponsive to the amplified front facet monitor signal to produce thegain adjusted front facet monitor signal.
 8. The apparatus of claim 6further including delay adjustment means for adjusting the front facetmonitor signal and the first and second data signals so that a phase ofthe front facet monitor signal substantially matches the first andsecond data signals.
 9. The apparatus of claim 7 further including anamplifier responsive to the gain adjusted front facet monitor signal todouble the gain of the gain adjusted front facet monitor signal.
 10. Theapparatus of claim 6 wherein each of the first and second gain adjusteddata signal producing means includes:(i) preamplifier means responsiveto the first and second data signal producing means to produce first andsecond amplified data signals, respectively; (ii) variable gainamplifying means responsive to the first and second amplified datasignals to produce first and second gain adjusted data signals,respectively; (iii) error detecting means responsive to the gainadjusted front facet monitor signal and to the first and second gainadjusted data signals to produce first and second feedback signals,respectively; and (iv) means responsive to the first and second feedbacksignals to produce the first and second gain adjusted data signals,respectively.
 11. Apparatus for reading data from a write-once opticalmedium with reduced bandwidth, comprising:(a) front facet detector meansfor producing a front facet monitor signal from a laser light beam; (b)first and second detector means for producing first and second datasignals, respectively; (c) first gain control means responsive to thefront facet monitor signal to produce a gain adjusted front facetmonitor signal; (d) second gain control means responsive to the firstdata signal and the gain adjusted front facet monitor signal to producea first gain adjusted data signal; (e) third gain control meansresponsive to the second data signal and the gain adjusted front facetmonitor signal to produce a second gain adjusted data signal; (f) meansresponsive to the first and second gain adjusted data signals to producea combined data signal as a function of the sum of the first and secondgain adjusted data signals; and (g) means responsive to the gainadjusted front facet monitor signal and the combined data signal toproduce a write-once data signal as a function of the difference betweenthe gain adjusted front facet monitor signal and the combined datasignal.
 12. The apparatus of claim 11 wherein the gain adjusted frontfacet monitor signal producing means includes:(i) a preamplifierresponsive to the front facet monitor signal to produce an amplifiedfront facet monitor signal; and (ii) a variable gain amplifierresponsive to the amplified front facet monitor signal to produce thegain adjusted front facet monitor signal.
 13. The apparatus of claim 11further including delay adjustment means for adjusting the front facetmonitor signal and the first and second data signals so that a phase ofthe front facet monitor signal substantially matches the first andsecond data signals.
 14. The apparatus of claim 12 further including anamplifier responsive to the gain adjusted front facet monitor signal todouble the gain of the gain adjusted front facet monitor signal.
 15. Theapparatus of claim 11 wherein each of the first and second gain adjusteddata signal producing means includes:(i) preamplifier means responsiveto the first and second data signal producing means to produce first andsecond amplified data signals, respectively; (ii) variable gainamplifying means responsive to the first and second amplified datasignals to produce first and second gain adjusted data signals,respectively; (iii) error detecting means responsive to the gainadjusted front facet monitor signal and to the first and second gainadjusted data signals to produce first and second feedback signals,respectively; and (iv) means responsive to the first and second feedbacksignals to produce the first and second gain adjusted data signals,respectively.